1. Field of the Invention
The present invention relates to a sintered electrically conductive oxide, to a thermistor element employing the oxide, and to an apparatus employing the thermistor element such as a temperature sensor.
2. Background Art
Conventionally known thermistor elements include those which employ a sintered electroconductive oxide whose electric resistance varies with temperature. One use of such a thermistor element is to measure the temperature of exhaust gas discharged from an internal combustion engine such as an automobile engine. In such a use, as a result of recent enhancements in the precision of exhaust gas purification systems, there has been an increased demand for thermistor elements having good heat resistance in a high-temperature region near 900° C. Meanwhile, in order to detect a failure (wire breakage) of a temperature sensor in an on-board diagnostic system (i.e., an OBD system) or the like, there is a need to detect the temperature of an engine even when its temperature is low, for example, when the engine is started or when an engine key is in its ON state. Since the temperature at the start of the engine may be below zero particularly in a cold district, there is a demand for a thermistor element which can carry out temperature measurement even at −40° C.
Patent Document 1 discloses a thermistor element which employs a sintered electroconductive oxide having a perovskite type crystal structure represented by the compositional formula: M1aM2bM3cAldCreOf (wherein M1 represents at least one of Y, Nd and Yb; M2 represents at least one of Mg, Ca and Sr: and M3 represents at least one of Mn and Fe). In this thermistor element, the sintered electroconductive oxide exhibits stable thermistor performance over a wide temperature range from −40° C. to 900° C., whereby the temperature within the temperature range can be appropriately measured.
Patent Document 1: Japanese Patent No. 5053563
3. Problems to be Solved by the Invention
The present inventors found that the sintered electroconductive oxide disclosed in Patent Document 1 has a problem requiring improvement. Specifically, in conventional sintered electroconductive oxides having a perovskite type crystal structure, the B site element(s) determine the electrical conductivity of the oxides. In the perovskite type crystal structure disclosed in Patent Document 1, the B site atom includes (Mn, Fe) as M3, Al, and Cr. Among these elements, Al has a consistent valence of +3. Thus, the conductivity is mainly determined by (Mn, Fe) as M3 or Cr. Furthermore, in Patent Document 1, the factor (atom fraction) c of element M3 is 0.150 to 0.600, and that of factor e of Cr is 0.005 to 0.050. This indicates that (Mn and Fe) as M3 mainly determine electrical conductivity. However, Mn and Fe are elements which readily undergo valance change. Thus, when the oxide is subjected to a temperature of higher than 900° C., the temperature characteristics of the sintered electroconductive oxide vary. Consequently the oxide may fail to meet the aforementioned recent requirement of high heat resistance.